1. Field of the Invention
The present invention is generally concerned with printing machines including a turret, which is mounted to rotate about its axis and which carries, radially, at its periphery, a plurality of object-carrier supports which are themselves mounted to rotate on the turret, and a plurality of workstations which are circumferentially distributed around the turret, in line with the path of the object-carrier supports, and some of which are printing stations while others are processing stations such as, for example, drying, varnishing, flame treatment, dust removal or other stations.
2. Description of the Prior Art
Such printing machines are used to print circular objects, for example, such as flexible tubes, for example, made of synthetic and other materials and adapted to be printed with several different colors in succession.
Whether the printing stations are screenprinting or offset printing stations, for example, or gold blocking printing stations, it is important for the object-carrier supports to maintain the same orientation from one to the other so that at each of them the printing is properly indexed relative to a common reference.
To this end, the object-carrier supports are usually each constrained to rotate with a gear that meshes with a central ring common to all of them.
This arrangement, which has the advantage of simplicity, because it provides the required synchronization mechanically, has the drawback of entailing identical rotation of the object-carrier supports at all the workstations, i.e. not only at the printing stations but also at any processing stations between the printing stations.
When the turret advances by one step from one workstation to the other, the object-carrier supports are inevitably subject to differential rotation movement because their gears mesh with the central ring.
Although it is possible to take account of this differential rotation movement in indexing the printing, at processing stations, such as ultraviolet radiation drying stations, for example, which require the objects to rotate through more than one revolution, there can be a risk of untimely overexposure of the objects to a greater or lesser degree on one side and underexposure to a greater or lesser degree on the diametrally opposite side.
A different approach has therefore been proposed, in order to retain the orientation of the object-carrier supports from one workstation to another, in particular in U.S. Pat. No. 3,253,538.
To be more precise, it has been proposed to associate a rotation drive finger with each object-carrier support, at an eccentric position relative to its axis, and to equip the printing machine with a fixed plate parallel to the turret and having at its periphery a groove in which the rotation drive fingers of the object-carrier supports are engaged. Rotation drive units which interrupt the groove locally, from place to place, are mounted to rotate under the control of drive means. Some of the drive units are each individually associated with printing stations and others are each individually associated with processing stations. Each drive unit includes a groove in which the rotation drive fingers of the object-carrier supports are successively engaged turn and turn about.
Accordingly, there is no differential rotation movement of the object-carrier supports between the workstations.
However, at present, the drive means are common to all the rotation drive units so that, as previously, the rotation of the object-carrier supports is the same at all the workstations, whether they are printing stations or processing stations.
It is desirable for different rotations to be possible at some workstations at least.
A general object of the present invention is an arrangement that satisfies this requirement.